'Let's get the sex out of the way," says Lisa Hilton, dropping her voice to a dramatic whisper as she leans in with a playful smile over tea and scones at the Merrion Hotel. "People say. 'Oh, it's very graphic, it's very shocking'," she reflects of the reaction to the numerous sex scenes in her new book, Maestra. "Have they seen what teenagers write to each other on Snapchat? Have they seen Tinder? Have they seen how quickly a conversation can progress from, you know, 'I like your photo' to '[something very sexually graphic]?' Sometimes that's three messages. Millions and millions of those messages are exchanged everyday."

'Let's get the sex out of the way," says Lisa Hilton, dropping her voice to a dramatic whisper as she leans in with a playful smile over tea and scones at the Merrion Hotel. "People say. 'Oh, it's very graphic, it's very shocking'," she reflects of the reaction to the numerous sex scenes in her new book, Maestra. "Have they seen what teenagers write to each other on Snapchat? Have they seen Tinder? Have they seen how quickly a conversation can progress from, you know, 'I like your photo' to '[something very sexually graphic]?' Sometimes that's three messages. Millions and millions of those messages are exchanged everyday."

Having secured a deal with Colombia Pictures, Maestra is already being touted as the new Fifty Shades of Grey - it's not, being a thriller which is far cleverer and immeasurably more skilfully executed. It tells the story of Judith, a working-class young beauty trying to make a career in the high-end London art world, who embarks on something of a murderous rampage around Europe. She also happens to like sex, a fact which is graphically portrayed in the book.

Such is the furore over the content of the book, that both Hilton's father and her uncle were recently doorstepped by paparazzi. "My dad was on his way back from the supermarket," she says with a baffled laugh. "It's astonishing, really. I couldn't work out why anyone would be interested. Maybe it was a slow week," she speculates. "I think it's quite a Marmite-y book. I think people either really like it or really dislike it."

In part, she feels, the reaction is partly down to Judith's unapologetic enjoyment of sex. "Describing orgasms is pretty hard actually," she says. "And then I thought, you know, I'm not going to try and think of waves crashing on shores. Kaleidoscopes of ribbons twirling in my brain. Fuck that. You go have an orgasm, ok? It happens. I think there's an element, which perhaps people found unnerving, that women aren't supposed to think like that. We're supposed to be romantic and emotional about sex. I think fiction has got round to the idea that women might enjoy it. But it's got to be in the context of some sort of sweeping passion. The idea that you could sort of get yourself off and then get on with your day? I think we see it onscreen more than we do in books. I think books are a little bit more old-fashioned in this respect." With Judith, she says she wanted to create a young woman who is modern.

One of the key set pieces of the book is an orgy Judith attends in Paris. Among other research Lisa carried out for the book - interviewing a member of the British army about guns, hedge funders in Geneva, semi-choking herself with a sanitary towel to investigate the logistics of one murder scene - she visited a similar party in Paris. "I approached it in the same way I approach all my research, and that's been a good hold-over from doing the history stuff for so many years. I think you've got to be thorough" she explains in brisk, no-nonsense tones. "I went to a famous place in Paris, I got a male friend to accompany me because they only admit couples. And yes, it was an eye-opener."

Gaining access wasn't hard, she says. "No no, you turn up. I think they have a bit of a door policy. I think if you're ugly you've got to be really rich. It's a well-known secret in Paris. I've got lots of friends who've been there. Not at all what I expected. I'd imagined something a bit sleazy, lots of very sexually dressed women wandering round in whatever. And it wasn't. It was actually like being in a rather chic bar. The women were well dressed. They weren't overly made up. They weren't tarty looking. They looked elegant, they looked normal. What was also very, very interesting to me was how the sex activity was about women. It was very much about women consenting. There was a whole sign language which was used in certain areas of the club whereby women indicated whether they did or didn't want something to happen. It seemed to me that it was about women's pleasure a great deal more than I'd anticipated. I found it fascinating. You can participate as much or as little as you choose. There was no sense in which I was peeping over the partition, I didn't find it uncomfortable at all. I think I'm quite a sexually open person in my views.

"She's very bored by the idea of women as victims," says Hilton, an Oxford graduate until now known for her historical biographies, of her heroine. "This idea that women can't be sexually active or sexually frank, unless they've been traumatised, or unless there's some sort of deep seated emotional need that's not been met. Well, no, she just likes fucking, that's it. It's what she likes to do."

It's not the first time Hilton has been at the centre of a something like this. In 2007 she wrote an article for The Guardian on cheating, outlining her own infidelities. "Can I tell you that I wrote that when my daughter was a year old?" she says now with a frustrated rolling of the eyes, of the piece which for years was one of the first things to pop up in a Google search of her name. "The thing about being a feature writer, is that your editor tells you what they want, and you do it. I think being a feature writer is a bit like playing a character. That article in no way reflects my feelings on the matter, then or now. You tell a story, and if you tell it well it will get up people's noses. It has haunted me, that blasted article. I still get letters about it now. I wrote it because I needed to pay the rent."

Now though, she's fairly sanguine about the whole thing, although she says she made the mistake of looking at her reviews on Amazon, something she says she'll never do again. "I've never done it with any other book. I've been really good about this one, I haven't looked at a thing. And then last night, someone told me 'Oh, we've had some really good social media.' I don't do social media. I thought I'll have a look on Amazon. Well, I wish I hadn't. Because quite a lot of the bad ones are saying well she just changes character too many times. She changes roles too easily. And I just thought, 'Yeah, well, that's the point'. The point is she's not a fixed entity. She's fluid, and she reinvents herself like a chameleon according to need and according to circumstance."

While a physical description of Judith is never given in the book - Hilton wanted the reader to project their own ideas, and she thinks the film role should go to an unknown - the 40-year-old author herself is a petite, stunning blonde, lightly tanned, elegantly dressed in Isabel Marant. She speaks in soft, precisely clipped tones, not a trace of her northern beginnings remain.

Born in Liverpool and later moving to the rural South-West of France with her mother after her parents separated, she returned to England to study at Oxford, then later art in Florence and Paris, before working in a London auction house. "I was a professional published writer at the age of eleven" she explains with a wry smile. Her grandmother used to buy her Jackie magazine, and, concluding that the romantic stories were "pretty formulaic," she wrote one, by hand, and sent it in. They printed two, for which she received cheques of £50.

"So there was never any doubt in my mind that this was going to be my job. But I didn't do any of the things you're supposed to do," she says with a laugh. "I didn't write anything else again until I was about 24. I didn't do a creative writing course." She studied English at Oxford, but didn't write for the student newspapers while there. "I couldn't be arsed, to be honest. I just thought this is what I'm going to do. I just didn't feel the need to go on about it."

Doing her masters in Paris, she decided to try her hand at writing a book, what later became her biography of Louis XIV's mistress Athenais de Montespan, Athenais: The real queen of France. "Evelyn Waugh said if you want to be a novelist you should write biographies first because it's the best discipline and training. So I had a go at writing my first biography. And that's kind of the only job I've ever had really." She writes at home at the kitchen table, in the London home she lives in with her now ten-year-old daughter.

Becoming a mother did not make working from home more challenging, she says. "Weirdly not, because it gave me so much more drive. I had a book on commission when I was pregnant. I had to start when she was five weeks old, otherwise I wouldn't have met the deadline. So I only had a very short time to work each day. And I kind of had an excuse for not having a social life for two years, because I had a small child. So although, in a way, yes I was so tired, I think it actually gave me a lot of focus. I mean I went to the gym, I looked after my daughter, I wrote my book and I went to bed at nine o'clock at night. That was it. That was all I did for two years. I remember once asking the historian Antonia Fraser, she's a good friend, and she's got six children, how on earth did she do it. And she said, 'Well, you know, writer's block is an indulgence only male writers can afford. You just haven't got time'."

At the time of her daughter's birth she was living in Italy with her then husband, an Italian composer; an article outlining why the dolce vita is in fact a myth, was another one which raised a few eyebrows. Two years ago they divorced after 12 years of marriage.

"But actually, I like him much better as a husband now I'm not married to him," she says with a smile. "And we're collaborating on a project together this year." They've written an opera together, he as composer, she writing the words. It premieres in Milan in June, before moving to New York in October. "So that's actually really nice. It's obviously very sad when a marriage doesn't work out, but I think we've been really lucky."

"Marriages?" Hilton says with a questioning laugh when I enquire about other marriages. "I did get married to my French boyfriend on a beach in Florida when I was 19," she concedes in a dismissive tone that suggests 'but of course that is of no consequence'. "It was one of those things where you wake up the next morning and think, 'Oh God, what have I done. What will we do, we must get divorced.' So then we went to Mexico and got a divorce. So I'm not sure that really counts."

Maestra first came about when, a few years ago, her then agent asked her to have a go at something erotic. "I think it was in the aftermath of that book," she says, referring to EL James's Fifty Shades of Grey. "I couldn't quite understand why, but I thought, 'Okay, sure, have a crack'." Her agent hated the short first draft.

Finding herself at a loose end one summer between book contracts, she decided to combine what she had written with an old manuscript begun years previously when working in the auction house. "Dreadful thing really. If you can imagine Bridget Jones goes to Sotheby's," she says briskly.

"So I started it, and it just really made sense to me as a book. Maybe because it was the first thing I'd written for pleasure for a long time, it went quite quickly. I mean, I'm not a person who enjoys writing. I hate it. Staring at it and thinking, 'Oh God, it's boring, it's crap, I'm boring, I'm crap. What time is it? Have I really got to do another 1700 words? An ongoing conversation with one's own inadequacies."

Finishing the book, she sent it back to her agent, who still hated it. As did every other publisher she showed it to. Instead of being deterred however, she decided there was something in this. "I think if it hadn't produced such extreme reactions I'd have put it to bed. It was the fact that it seemed to really get up people's noses."

She actually bought a guide to publishing your own e-book, an event she laughingly described over dinner one night in a friend's restaurant. The friend kept the manuscript, and gave it to a famous publisher who happened to be one of her regular diners, saying, "I'm putting it on your plate and you can't have any dinner until you start reading it." When she opened the restaurant the next morning he was waiting on the pavement outside. "Six weeks later I'm sitting on the plane on my way to the Sony studios in Hollywood with my publisher, and we're both thinking 'is this real?' Has there been a mistake? And that's very much how I still feel," she laughs.